Discussion on Metallogenic Epoch of Tuwu and Yandong Porphyry Copper Deposits in Eastern Tianshan Mountains, Xinjiang
Rui Zongyao, Wang Longsheng, Wang Yitian (Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China) Liu Yulin (Department of Geology, Peking University, Beijing 100871, China)
With regard to the metallogenic epoch of the Tuwu and Yangdong porphyry copper deposits in Eastern Tianshan Mountains, the previous researchers held that they were of early Carboniferous-Permian according to the fossil organism discovered in the sedimentary rocks about 1km away from the ore district and the isotope ages of the granitoids adjacent to the ore district. In addition, the tectonic setting of the copper mineralization was inferred to be of the active continental margin of Kazakhstan plate-the Carboniferous volcanic island arc. However, the suggestions mentioned above need to be reconsidered according to the new evidence of isotope chronology obtained in this study: (1) the age of the plagiogranite porphyry (ore-baring porphyry) is about 369～356Ma (Rb-Sr isochron dating and single-zircon U-Pb dating), indicating that the porphyry occurred at the end of Devonian; (2) the age of molybdenite is (323±2)Ma (Re-Os isochron dating), suggesting its formation in early Carboniferous; (3) the age of the altered rock is about 341～310Ma (K-Ar dating), implying that the alteration must have taken place in early Carboniferous; (4) the ages of ore-bearing volcanic rocks (named diorite porphyrite before) range from 416Ma to 360Ma, which should be assigned to Devonian; and (5) the tectonic setting of porphyry copper deposits in Eastern Tianshan Mountains belongs to the active continental margin of the Tarim Plate, and hence the deposits are likely to be products of the island arc volcanism-plutonic activity.